Formula to output future date

I’m trying to create a formula to do the following:

Invoice Date (01/01/2024) + Contract Length Months (12) = Contract Renewal Date (01/01/2025)

I have the following properties set up on a deal which is:
Invoice Date = Date

Contract Length Months = Number

Renewal Date = Date

I have tried the following formula, but it won’t work:

plus_time([invoice_date],[contract_length_months], “months”)

If I change [contract_length_months] to a number such as 12 so it looks like this:

plus_time([invoice_date],12, “months”)

It works. It generates the future date. I can’t set the value as it needs to come from the field as it varies per deal.

Any help would be great! Thanks

This is now resolved